Implement to output the graph after the computation. It is modeled
directly after the Hadoop OutputFormat.
ImmutableClassesGiraphConfiguration is available
It's guaranteed that whatever parameters are set in the configuration are
also going to be available in all method arguments related to this output
format (context in createVertexWriter, checkOutputSpecs and
getOutputCommitter; methods invoked on VertexWriter and OutputCommitter).
So if backing output format relies on some parameters from configuration,
you can safely set them for example in
DefaultImmutableClassesGiraphConfigurable.setConf(org.apache.giraph.conf.ImmutableClassesGiraphConfiguration).

checkOutputSpecs

Check for validity of the output-specification for the job.
(Copied from Hadoop OutputFormat)

This is to validate the output specification for the job when it is
a job is submitted. Typically checks that it does not already exist,
throwing an exception when it already exists, so that output is not
overwritten.